Odd carburettor response
 

Hi all,
I was tinkering with the mixture on my 1968's 1800S twin HD6 SU carburetors. I was enriching the mixture by one flat at a time to get to maximum revs. After each adjustment I would blip the throttle and wait for the carbs to settle back to tickover (around 800 rpm). Several times the tickover dropped to 650rpm and stayed there till I blipped the throttle and the revs returned to around 800rpm. Tried turning off the engine and restarting and the same thing happened again.

Given that I was only adjusting the mixture I assumed it must some how be related to the pistons not rdropping correctly. Am I right in thinking that unusual fall off in revs indicates that would the carbs were temporarily too lean (too much air entering the carb?). I removed the dashpots and checked the needles which looked fine, made sure everything was nice and clean and checked the dash pot oil level. I also checked that the pistons were centered and making a nice clunk when lifted and dropped.

I rebuilt the carbs about 4 years ago.

I am at bit of a loss what to try next.

Any ideas?

Phil

fb;

I don't quite agree with a couple of assertions in your posting...

"...enriching the mixture by one flat at a time to get to maximum revs"...all other things being equal and untouched, I would expect leaning mixture to raise revs to a point...

"Several times the tickover dropped to 650rpm and stayed there till I blipped the throttle and the revs returned to around 800rpm." I would actually expect such an action.

"...mixture...related to the pistons not dropping correctly." ...height position of Dashpots is a result of amount of ingested air determined by Throttle. If Dashpots are free and clear and lubed, they will seek their appropriate height, and that will result in correct Metering Needle exposure and mixture. Base level of Jet sets mixture and I would not expect that to need more than a couple of flats of seasonable adjustment once set and checked with a roadtest.

"...what to try next." ...and you haven't given a reason or problem or symptoms why you are "tinkering with the mixture" in the first place...maybe you should start there!

...but first, may I suggest you have a look at the (Superb!) John Twist videos for a proper understanding of the function of your HS6 carbs: SU Part 1. https://www.youtube.com/watch?v=7GRAcqDySog SU Part 2. https://www.youtube.com/watch?v=60Bj_2cZQnc

Cheers

Hi Ron,

Thanks for the reply.

The questioned I should have asked is "what causes the idle to drop by 150rpm with no change made to the settings on the carburetor?" It was unexpected to me.

I tend to approach mixture setting from the lean side (which you also suggest on your web page) with a initial setting of the jet 50 thou below the bridge. I have always found that from this setting that as I turn the adjusting nut down to enrich the mixture the revs increase to a max before falling off again. I am trying to find the "fastest idling speed consistent with even running" as the SU Tuning Manuals suggests.

"Tinkering" with the carbs is one of the pleasures of owning a SU carburetored car. When you get the tuning right it enhances the whole driving and owning experience. Carburetors and points every time for me. Fuel injection and electronic ignition for those who don't like to "tinker".

I've watched the John Twist videos many time and they are a great resource. Not sure that I ever remember hearing him say that having the idle drop by 150 rpm was "normal" but I'll certainly take another look.

Thanks for your comments.

Phil

idle speed
 

I have found in the past that sticking advance weights in the distributor can cause a variation of the idling speed,might be worth a check.